Various types of electrochemical sensing elements are known; one such sensor has an ion conductive solid electrolyte which, however, to provide useful sensing output function, is operative only when its temperature is above about 400.degree. C. Only then will it have sufficient ion conductivity to permit its practical use. If the test gases, for example the exhaust gases of an internal combustion engine, contain lead, then the temperature may have to be raised to above 500.degree. C. before the output signal from the sensor will be essentially independent of the lead content in the gas. The output signals of such sensors are, additionally, highly dependent on the temperature of the solid electrolyte itself.
It is desirable to decrease the response time of such sensors when exposed to cool gases, in order to obtain an output signal which can be readily evaluated. It is also desirable to increase the lifetime of such sensors, particularly when exposed to exhaust gases which contain lead, and further to increase the accuracy of measurement, when exposed to such lead-containing gases. To improve the operation of such sensors, then, it is desirable to provide controlled heating elements for the sensors. Controlled heating elements are also desirable if such sensors are located in the exhaust system of the internal combustion engine at locations in which the exhaust gases are already quite cool.
A sensor of the type to which the present invention relates is described in U.S. Pat. No. 3,546,086; this sensor has a solid electrolyte tube, closed at one end, within which a resistance heating element and a temperature sensing element are located. Such sensors, while operating satisfactorily, are expensive to manufacture, particularly under high volume conditions, and are difficult to make in mass production. There is always room for increasing the shock and vibration resistance of such sensors. Other types of sensors to which the present invention can be applied are electrochemical sensors with ion conductive solid electrolyte on which a protective layer is applied at the side facing the test gases--typically exhaust gases from an internal combustion (IC) engine, and on which further an electron conductive catalyzing layer is provided. Such sensors are described, for example, in U.S. Pat. No. 3,645,875. The sensors may, additionally, be supplied with a diffusion layer which limits the current of the sensor, see, for example, U.S. Pat. No. 3,691,023, U.S. application Ser. No. 6,093, filed Jan. 24, 1979, DIETZ (Continuation-in-Part of U.S. Ser. No. 885,368, filed Mar. 13, 1978, now abandoned).